There is growing interest in high-temperature-capable semiconductor electronics for various applications, not least of which are electric power conversion and sensing in fuel-burning and other high-temperature environments. Silicon carbide (SiC) has emerged as an especially promising semiconductor electronic material because of its high thermal, mechanical, and electrical strength, because it has favorable properties as a switching material in transistors, and because it is compatible with CMOS processes.
Among semiconductors, only diamond has a greater mechanical strength than SiC. Crystalline silicon carbide maintains its structural integrity to temperatures above 2500° C.; at its high-temperature limit it sublimates rather than melting.
There has also been growing interest in the use of piezoelectric devices based on microelectromechanical systems (MEMS) technology for sensing and signal processing. In such devices, a piezoelectric element transduces signals between the mechanical or acoustic domain and the electrical or radiofrequency domain.
Piezoelectric thin-film aluminum nitride (AlN), in particular, has been recognized as a promising piezoelectric transducer material that is stable at high temperatures. Aluminum nitride can be used for piezoelectric transduction even at temperatures as high as 1000° C. or more.
In fact the piezoelectric properties of AlN persist up to 1150° C. It also has a coefficient of thermal expansion that closely matches that of silicon carbide. That coincidence offers the possibility to monolithically integrate AlN-based MEMS with SiC-based CMOS circuitry without risking degradation due to thermal mismatch, even at high temperatures.
The monolithic integration of MEMS with SiC-based CMOS is not without difficulties, however. This subject has been discussed by, among others, M. B. J. Wijesundara and R. G. Azevedo, Silicon Carbide Microsystems for Harsh Environments, Springer (2011), Section 6.1.2, pages 191-195.
As Wijesundara et al. point out, each step of the process flow must take into account the thermal budget and the chemical compatibility of the previous layers.
The same authors discuss three alternative schemes for hybrid integration, which they refer to as post-CMOS MEMS insertion (in which the MEMS circuit is fabricated last), interleaved CMOS insertion, and pre-CMOS insertion (in which the MEMS circuit is fabricated first).
The post-CMOS scheme is the most attractive, because it permits the MEMS structures to be fabricated on top of the CMOS integrated circuit. However, the authors note that the MEMS process will include deposition or annealing at temperatures outside the thermal budget of the underlying CMOS circuit. With conventional silicon-based CMOS, the MEMS process temperatures are therefore capped at 400-500° C. Even if the CMOS is based on silicon carbide, the process temperature must be limited to avoid degradation of metal contacts.
Thus there remains a need for new methods of post-CMOS fabrication of monolithically integrated hybrid devices that are high-temperature capable.